Some US scientists warned the full-body, graphic-image X-ray scanners now being used to screen passengers and airline crews at airports around the country may be unsafe.

"They say the risk is minimal, but statistically someone is going to get skin cancer from these X-rays," Dr Michael Love, who runs an X-ray lab at the department of biophysics and biophysical chemistry at Johns Hopkins University school of medicine, told AFP.

"No exposure to X-ray is considered beneficial. We know X-rays are hazardous but we have a situation at the airports where people are so eager to fly that they will risk their lives in this manner," he said.

The possible health dangers posed by the scanners add to passengers' and airline crews' concerns about the devices, which have been dubbed "naked" scanners because of the graphic image they give of a person's body, genitalia and all.

The Transportation Security Administration (TSA) began rolling out full-body scanners at US airports in 2007, but stepped up deployment of the devices this year when stimulus funding made it possible to buy another 450 of the advanced imaging technology scanners.

Some 315 "naked" scanners are currently in use at 65 US airports, according to the TSA.

Passengers and airline crew members, including pilots, are randomly selected to pass through the scanners. They have the option of refusing, but will then be subjected to what the TSA calls an "enhanced" manual search by an agent.

"People are not reacting well to these pat-downs," said a travel industry official, who asked not to be named.

Government officials have said that the scanners have been tested and meet safety standards.

But Captain David Bates, president of the Allied Pilots Association, which represents pilots at American Airlines, urged members to avoid the full-body scanner.

"No pilot at American Airlines should subject themselves to the needless privacy invasion and potential health risks caused by the body scanner," he said in a letter this month, which was obtained by AFP.

"Politely decline exposure and request alternative screening," even if "the enhanced pat-down is a demeaning experience," he said.

A group of scientists at the University of California, San Francisco (UCSF) raised concerns about the "potential serious health risks" from the scanners in a letter sent to the White House Office of Science and Technology in April.

Biochemist John Sedat and his colleagues said in the letter that most of the energy from the scanners is delivered to the skin and underlying tissue.

"While the dose would be safe if it were distributed throughout the volume of the entire body, the dose to the skin may be dangerously high," they wrote.

The scientists say the X-rays could pose a risk to everyone from travelers over the age of 65 to pregnant women and their unborn babies, to HIV-positive travelers, cancer patients and men.

"Men's sexual organs are exposed to the X-rays. The skin is very thin there," Love explained.

The Office of Science and Technology responded this week to the scientists' letter, saying the scanners have been "tested extensively" by US government agencies and were found to meet safety standards.

But Sedat told AFP Friday: "We still don't know the beam intensity or other details of their classified system."